1. Field of the Invention
This invention relates to an iron nitride magnetic powder suitable for constituting the magnetic layer of a high recording density medium and a method of producing the powder.
2. Background Art
In order to achieve the increasingly higher recording density required by today's magnetic recording media, efforts are being made to enable use of shorter recording wavelengths. For this, it is necessary to make the magnetic particle size much smaller than the length of the region for recording the short-wavelength signal. If it is not, a distinct magnetic transition cannot be produced, making practical recording impossible. The particle size of the magnetic powder is therefore required to be sufficiently small.
To realize higher recording density, the resolution of the recording signal must be increased. Reduction of magnetic recording medium noise is therefore important. Noise is largely attributable to particle size. The finer the particles, the lower the noise becomes. This also makes it necessary for a magnetic powder used for high density recording to have sufficiently small particle size.
Moreover, a magnetic powder used in a magnetic recording medium enabling high density recording requires high coercive force (Hc) in order to maintain magnetism in the high-density medium and to ensure the output. In addition, coercive force distribution (called Switching Field Distribution: SFD) must be made as narrow as possible because a smaller coercive force distribution range is essential for realizing high-density recording.
Even if such a magnetic powder should be obtained, various problems will nevertheless arise if the thickness of the magnetic layer obtained by applying the powder in the form of a coating material is too thick. This is because self-demagnetization loss, thickness-loss attributable to magnetic layer thickness and other such problems that are not major issues when using conventional long recording wavelengths come to have a pronounced effect and give rise to phenomena that, for instance, make it impossible to realize sufficient resolution. Such phenomena cannot be eliminated only by enhancing the magnetic properties of the magnetic powder or improving surface properties by application of medium production technologies. Magnetic layer thickness reduction is essential. The degree of magnetic layer thickness reduction that can be achieved when a conventional powder having a particle size of around 100 nm is used is limited, so that small particle size is also essential in this aspect.
However, when particle refinement reaches the point that the decrease in particle volume exceeds a certain degree, a marked degradation of magnetic properties occurs owing to thermal fluctuation, and when particle size decreases still further, superparamagnetism is exhibited and magnetism ceases to be exhibited. Another problem is that the increase in specific surface area with increase in particle size refinement degrades oxidization resistance. From this it follows that a magnetic powder suitable for use in a high-density recording medium requires thermal stability enabling it to resist superparamagnetism even when refined, i.e., must be capable of achieving a large anisotropy constant, high Hc, high σs, low SFD and good oxidization resistance, and must be composed of particles fine enough to enable very thin coating. No magnetic material having these properties has been put to practical use heretofore.
JP2000-277311A (Ref. No. 1) describes an iron nitride magnetic material of large specific surface area that exhibits high coercive force Hc and high saturation magnetization σs, and teaches that excellent magnetic properties can be achieved irrespective of shape owing to a synergistic effect between the crystal magnetic anisotropy of an Fe16N2 phase and powder specific surface area enlargement.
JP2001-176715A (Ref. No. 2) describes a low-cost magnetic material exhibiting high saturation magnetization σs as a magnetic powder in which 10–90% of Fe16N2 phase is generated and particularly teaches that the saturation magnetization is maximum when the Fe16N2 phase generation rate is 60%.
As improvements on the magnetic powder of Ref. No. 1, WO 03/079332 A1 (Ref. No. 3) proposes rare earth element-iron-boron system, rare earth element-iron system and rare earth element-iron nitride system magnetic powders composed of substantially spherical or ellipsoid particles and states that a tape medium produced using such a powder has excellent properties, that, in particular, the rare earth element-iron nitride system magnetic powder whose main phase is Fe16N2 is high in saturation magnetism despite being composed of 20 nm particles and also good storage stability because it has a high coercive force of 200 KA/m or greater and a small BET specific surface area, and that use of this rare earth element-iron nitride system magnetic powder enables a dramatic increase in the recording density of a coating-type magnetic recording medium.
This rare earth element-iron nitride system magnetic powder is produced by ammonia nitriding in which rare earth element-iron system magnetic powder obtained by reducing magnetite particles with a surface-adhered rare earth element is nitrided using ammonia gas. Although Ref. No. 3 is said to enable replacement of part of the iron in the rare earth element-iron nitride with another transition metal element, it is pointed out that a long time is required for the nitriding reaction when a large amount of cobalt is added.
JP-Hei11-340023A (Ref. No. 4) discloses the basic invention to obtain an iron-nitride magnetic powder of Fe16N2 phase by a low temperature nitriding method using ammonia gas.
Although Ref. Nos. 1–3 say that a fine magnetic powder with good magnetic properties is obtained when an Fe16N2 phase having large crystal magnetic anisotropy is generated, they do not say that a higher ratio of Fe16N2 phase is always better. For example, Ref. No. 2 states that the highest σs is obtained when the Fe16N2 phase ratio is about 60%. Moreover, regarding the ratio between Fe16N2 phase and α-Fe phase of the magnetic powder inner layer (core portion), Ref. No. 3 says that the core portion need not be entirely Fe16N2 phase but can be a mixed phase including α-Fe and also that the desired coercive force can be easily set by regulating the nitriding conditions. Still, although these conventional technologies present no major problem when only coercive force and saturation magnetization are taken into consideration, when they are considered from the viewpoint of powder coercive force distribution (called as Bulk Switching Field Distribution: BSFD) and tape coercive force distribution (called as Switching Field Distribution: SFDx) in the direction of tape orientation (defined as the “x direction”), the large difference in coercive force between Fe16N2 phase and each of α-Fe phase and Fe4N phase (which contains even more nitrogen than Fe16N2) causes such a mixed phase to have a broad SFD distribution including two or three maximum values.
The fact that a magnetic powder has a broad bulk switching field distribution means that high and low Hc particles are intermixed. Therefore, when such a magnetic powder is used in a coating material for preparing a tape to be used as a high-density recording medium, noise readily occurs. Moreover, when low Hc components are present, such particles are apt to experience erasure of recorded content because they cannot maintain magnetism owing to thermal fluctuation, so that a reliability problem arises. Therefore, a magnetic powder that is substantially Fe16N2 phase with no mixed in α-Fe phase, Fe4N phase or the like is preferable as a magnetic powder for a high-recording density magnetic medium. Although JP-Hei-11-340023A (Ref. No. 4) sets out a method of producing Fe16N2 phase particles by low-temperature nitriding, it is silent regarding the crystal state, coercive force Hc and switching field distribution of the product.
As regards the oxidation resistance of an iron nitride magnetic powder comprising Fe16N2, the rare earth element-iron system magnetic powder described in Ref. No. 3, for example, achieves Δσs of 12.6% in a 20 nm average particle diameter magnetic powder comprising a mixture of Fe16N2 and α-Fe phases and containing 5.3 at. % of Y, for example. However, this oxidation resistance probably needs to be increased to a still higher level because specific surface area increases markedly when further particle refinement is carried out. Thus there has not been known a method that can effectively improve the oxidation resistance of an iron nitride magnetic powder comprising Fe16N2 while maintaining its high Hc, high σs and low SFD unchanged.